This work experimentally studied the effects of ozonation treatment on waste sludge. During the treatment process, various parameters characterizing sludge were investigated. A substantial reduction in the volume of sludge and the release of intracellular and extracellular materials were observed. With the increase of ozone dose, the settleability and water content of sludge improved obviously, but the filterability of sludge deteriorated drastically. In addition, the evolution of particle size was evaluated, which proved the breakup of sludge flocs and cells. There existed a threshold of ozone dose which was 0.04 g O-3/ gMLSS in this work. Above the threshold, the soluble chemical oxygen demand ( SCOD), protein, carbohydrate, total nitrogen and total phosphorus in supernatant increased remarkably and the electron transport system ( ETS) sludge activity decreased. Organic nitrogen and organic phosphorus occupied the main part of total nitrogen and total phosphorus in the supernatant.